Looks like its wet in the pic you attached, the bare steel appears to be showing a bit. The finish is "phosphate" but to be more specific, Zinc Phosphate, which produces shades of grey. Color varies on these based on bath strength, time in solution, temperature of bath, and material finish. A fresh bath may produce a color some would call black. If you really want black (incorrect from my experience) use a fresh bath of Manganese Phosphate.
Phosphate coatings, zinc or manganese, are not particularly good corrosion inhibitors. They receive their corrosion resistance through a post plating oil dip, where the coating structure accepts and holds trace mounts of oil, the same reason you oil a gun surface. If you do the plating yourself you can use a sealer (a clear coat of sorts) that will not attract dust/dirt and will perform well as a corrosion inhibitor.
